BlinkGenPropertyTrees (BGPT) is an incremental step towards making
compositing decisions after the paint lifecycle phase. The primary
changes are to build property trees in blink and send a layer list to
the compositor, rather than building property trees in cc from a layer
tree. This patch marks the project as experimental which will give us
test and perf coverage on the bots.

Sheriffs: This shifts time (e.g., we now run PaintArtifactCompositor
and no longer run the cc property tree builder) and will likely change
performance benchmarks. Because this is an initial trial, feel free to
roll this patch out.

Bug: 836884
Change-Id: Ib867beebbead940162db0801822389f76e8eb8dc
Reviewed-on: https://chromium-review.googlesource.com/c/1294204Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602150}